A lithographic apparatus is a machine constructed to apply a desired pattern onto a substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). A lithographic apparatus may for example project a pattern from a patterning device (e.g. a mask) onto a layer of radiation-sensitive material (resist) provided on a substrate.
The wavelength of radiation used by a lithographic apparatus to project a pattern onto a substrate determines the minimum size of features which can be formed on that substrate. A lithographic apparatus which uses EUV radiation, being electromagnetic radiation having a wavelength within the range 5-20 nm, may be used to form smaller features on a substrate than a conventional lithographic apparatus (which may for example use electromagnetic radiation with a wavelength of 193 nm).
The small size of features patterned using EUV radiation means that any particulate contamination within a lithographic apparatus can have significant detrimental effects on a manufactured integrated circuit. For example, if a particle was present on a patterning device during patterning it may cause an image of the particle to be formed on a substrate. It is known to use a pellicle to protect a patterning device from particle contamination, so as to prevent any degradation in the performance of the lithographic apparatus in the presence of some particles. However, to form a pellicle which is sufficiently transparent to EUV radiation so as to not reduce the performance of the lithographic apparatus each pellicle must be made of an extremely thin film. Thin films tend to be flexible and have a tendency to deflect when exposed to pressure gradients, mechanical vibrations or mechanical stresses when in use or in transit. Any such deflection of a pellicle can cause the pellicle to come into contact with other components of the lithographic apparatus, which may cause damage to the pellicle, or degradation in patterning performance.
In known pellicles a support grid may be provided across the pellicle in order to prevent deflection of the pellicle. However, such a support grid may cause patterning of the EUV radiation.